This invention pertains to an improvement in the production of superplastically formed, diffusion bonded, metal alloy structures, and more particularly, to providing uninterrupted wall thickness at the re-entrant angle that occurs in the structure at the parting line between the two halves of the forming fixture.
Combined superplastic formed and diffusion bonded (SPF/DB) structures are now state of the art in production technologies in the aerospace world. Superplasticity may be defined as the property of some materials to develop unusually high tensile elongations with minimum necking.
Currently two different methods and structures of forming SPF/DB sandwich structures are popular. The first process was pioneered by C. H. Hamilton as taught in U.S. Pat. No. 3,927,817 and the second is an improved structure and process as taught by Hayase et al in U.S. Pat. Nos. 4,217,397 and 4,304,821, both of which are incorporated herein by reference. Both of these popular methods use a split forming die. The sheets to be superplastically formed are held between the two halves of the forming die so as to produce a completed part that has a flange around the perimeter. In the normal case the sheets being formed into both core sheets and face sheets are diffusion bonded by mechanical die pressure at the perimeter flange as a result of the process.
Inherent in the superplastic forming process is some variation in the wall thickness of the finished part which occurs for two reasons. One, strain is proportional to the stress and the part being formed is not uniformly stressed so it is not uniformly strained and two, when the sheet being formed contacts either the mold or another sheet being formed further strain at that point is inhibited because it immediately diffusion bonds and further deformation is restricted to the free region of the sheet. As shown in the section of FIG. 1, Prior Art, the radius of the corners of the fixture that the sheets are formed around needs to be reasonably generous to avoid too much thinning of the wall and as a result there is an entrance angle formed between the two inner sheets where they enter the flanged angle. Typically, as shown, that entrance angle is such that if it is necessary to cut the flange off of the formed part in order to complete the finished part you actually have an open space in the outer wall. This problem can be minimized by very precise control of the welds on the outer perimeter of the sheets to be formed in relationship to the forming die or the inside sheets can actually be folded over on themselves at the approximate location of this entrance angle so as to provide extra material. However, both of these methods are time consuming, expensive, and require precise placement of the part to the die.
It is an object of this invention to fill the void created at this entrance angle so that when the flange is removed there is a solid outer wall and to accomplish this during the forming operation and to do it economically.